In recent years, arrays of photosensors have been used for optical imaging applications. Both photodiodes and charge-coupled devices have been employed to collect the photogenerated carriers. In both types of devices, however, the spatial resolution obtainable may be limited by diffusion of the photogenerated carriers within the semiconductor itself, even if other components of the optical system are optimized and scattered light is reduced. This occurs because a carrier photogenerated under one sensor can diffuse a significant distance in the underlying semiconductor substrate to be collected in the space-charge region of a distant sensor, thereby giving a spurious signal. This type of interaction may be termed "crosstalk" by analogy with the unwanted interactions encountered in communication systems. Such optical crosstalk between photosensing elements limits the spatial resolution of the array.
In U.S. Pat. No. 4,025,943, entitled "Photogeneration Channel in Front Illuminated Solid State Silicon Image Devices" there is disclosed a scheme for reducing crosstalk in which a thin layer of one conductivity type is externally biased with respect to a substrate of opposite conductivity type to prevent carriers from migrating to distant photosensors. However, the use of this device is complicated by the need for an external bias supply. Additionally, the presence of layers of two different conductivity types over the entire surface makes this structure less compatible with the integration of other circuitry into the same semiconductor chip.